In the past three decades, electricity has risen from 25% to 40% of end-use energy consumption in the United States. With this rising demand for power comes an increasingly critical requirement for highly reliable, high quality power. As power demands continue to grow, older urban electric power systems in particular are being pushed to the limit of performance, requiring new solutions.
Wire forms the basic building block of the world's electric power system, including transformers, transmission and distribution systems, and motors. The discovery of revolutionary HTS compounds in 1986 led to the development of a radically new type of wire for the power industry; this discovery is the most fundamental advance in wire technology in more than a century.
HTS-coated wire offers best-in-class performance, carrying over one hundred times more current than conventional copper and aluminum conductors of the same physical dimension do. The superior power density of HTS-coated wire will enable a new generation of power industry technologies. It offers major size, weight, and efficiency benefits. HTS technologies will drive down costs and increase the capacity and reliability of electric power systems in a variety of ways. For example, HTS-coated wire is capable of transmitting two to five times more power through existing rights of way. This new cable will offer a powerful tool to improve the performance of power grids while reducing their environmental footprint. However, to date only short samples of the HTS tape used in the manufacture of next-generation HTS-coated wires have been fabricated at high performance levels. In order for HTS technology to become commercially viable for use in the power generation and distribution industry, it will be necessary to develop techniques for continuous, high-throughput production of HTS tape.
Vapor deposition is a process for manufacturing HTS tape where vapors of superconducting materials are deposited on a tape substrate, thereby forming an HTS coating on the tape substrate. Well-known vapor deposition processes that show promise for the high-throughput cost-effective production of HTS tapes include metalorganic chemical vapor deposition (MOCVD) and pulsed laser deposition (PLD). With the use of MOCVD or PLD processes, HTS film, such as yttrium-barium-copper-oxide (YBa2Cu3O7 or “YBCO”) film, may be deposited onto a heated buffered metal substrate to form an HTS-coated conductor. However, to date only short lengths of coated conductor wire samples have been fabricated at high performance levels with any of the above processes. Several challenges must be overcome in order to enable the cost-effective production of long lengths (i.e., several kilometers) of HTS-coated conductor.
One way to characterize coated conductors is by their cost per meter. Furthermore, cost and performance can be characterized as the cost per kiloamp-meter. More specifically, by increasing the current for a given cost per meter of coated conductor the cost per kiloamp-meter is reduced. This is stated as the critical current (Jc) of the deposited HTS material multiplied by the cross-sectional area of the film.
For a given critical current and width of coated conductor, one way to increase the cross-sectional area is by increasing the HTS film thickness. However, it has been demonstrated that with critical current as a function of thickness, the critical current may drop off and reach saturation as the thickness of a single layer of HTS film increases beyond approximately 1.5 microns. This is because beyond a film thickness of approximately 1.5 microns the HTS material becomes very porous, develops voids, and develops increased surface roughness, all of which contribute to inhibiting the flow of current. Since simply increasing the HTS film thickness does not result in a corresponding increase in critical current, a technical challenge exists in increasing the film thickness beyond 1.5 microns while also achieving a corresponding increase in critical current of an HTS-coated conductor in a cost-effective manner.
One approach to achieving high-quality YBCO thick films is to improve the morphology of the film, such as by increasing material density and smoothness, as the thickness exceeds 1.5 microns, thereby resulting in increased current capacity. Tatekawa, et al., U.S. Pat. No. 6,143,697, dated Nov. 7, 2000 and entitled “Method for Producing Superconducting Thick Film,” describes a method of producing a superconducting thick film that involves the steps of forming a thick layer comprising a superconducting material on a substrate; firing the thick layer formed on the substrate; subjecting the fired thick layer to cold isostatic pressing; and re-firing the thick layer subjected to cold isostatic pressing.
A drawback of Tatekawa, et al., is that while it is a suitable method for forming superconducting oxide thick films, it does not provide a cost-effective way to improve the morphology of the film and thus minimize the film defects, such as high porosity, voids, and surface roughness, and thereby provide thick HTS films having increased critical current. Tatekawa, et al., is therefore not suited for the cost-effective production of high-current HTS-coated conductors.
It is therefore an object of the invention to produce YBCO films with a thickness in excess of 1.5 microns with increased current capacity for use in the manufacture of high-current HTS-coated tape.